This invention relates to a motor drive circuit for a radio-controlled model, and more particularly to a motor drive circuit used for driving a motor of a model unit such as, for example, a model electric car.
A pulse stretcher circuit common to a servo IC which has been conventionally used for a motor controller amplifier is disclosed in Japanese Patent Publication No. 48352/1986 and constructed in such a manner as shown in FIG. 4.
More particularly, the conventional pulse stretcher circuit includes a first transistor 20 of which the emitter is grounded and a second transistor 21 of which the base is connected to the collector of the first transistor 20 and the emitter is grounded. A connection P between the collector of the first transistor 20 and the base of the second transistor 21 is grounded through a resistor R1 and a capacitor C1, and between the connection P and a power supply is connected a current source 22. Also, the pulse stretcher circuit includes a resistor connected between the collector of the second transistor 21 and the power supply.
In the pulse stretcher circuit constructed as described above, when a difference signal indicating the difference between a pulse signal fed from a receiver and a one-shot pulse formed within a motor controller is input to the first transistor 20, it stretches the difference signal over a period of time sufficient to cause a motor of a controlled mode to be actually driven, to thereby generate an output signal. More specifically, the variation of either the resistor R1 or the capacitor C1 permits the relationship between the difference signal and the output signal to be varied, so that the signal extended by the pulse stretcher circuit is subject to power amplification to drive the motor.
The pulse stretcher circuit described above employs the charge discharge characteristics of the capacitor to stretch the difference signal. Thus, the circuit permits the whole inclination of the input-output characteristics expressed when the pulse width of the difference signal is indicated on an X axis and the pulse width of the difference signal after it has been stretched is indicated on a Y axis to be freely varied. However, it fails to vary only the portion of the inclination within any specific range. For example, it fails to permit the pulse width of the difference signal to render only the portion of the inclination within a specific region steep or gentle.
This causes a manipulator to fail to sufficiently maintain a control region extending between an intermediate speed and a maximum speed which is most required during the controlling of a model electric car or the like, so that the conventional pulse stretcher circuit fails to exhibit satisfactory controllability when delicate controlling is required as in a race or the like.